1. Field of the Invention
The present invention relates generally to the reliability and restoration of optical transmission systems.
2. Related Art
Telecommunications networks that carry telephone calls and data include interconnected sites or nodes that process calls and route data. Optical transmission lines or links interconnect the nodes.
High speed data is modulated on light waves which are transmitted through the optical network. Any type of data can be carried over an optical link, including but not limited to speech, data input into or retrieved by a computer or computer database, and any digital data. Fiber optic cables carry far greater amounts of digital data than conventional electrical cables. A single optical channel operating at approximately 10 Gigabits/second (Gb/s) and transmitting data according to a high-speed synchronous digital hierarchy standard, such as the SONET OC-192 protocol, carries a data rate equivalent to 129,024 voice calls.
Multiple links are often employed between nodes to increase communications capacity and to provide back-up in the event of partial failures. The set of links interconnecting a given pair of nodes is referred to as a "span."
Further bandwidth improvement can be achieved by sending multiple modulated lightwave carriers at different frequencies through a single fiber. This technique is known as wavelength division multiplexing (WDM). Optical systems using WDM require optical transmitters and receivers that operate at different light wave frequencies. The optical transmission line, connecting an optical transmitter and receiver, can propagate many light wave signals of different frequencies simultaneously. For example, at least sixteen OC-192 channels can be carried on a single fiber pair within the so-called "erbium band. " A method and system for WDM is described in copending U.S. Application No. 08/923,461 entitled, "Method and System for Modular Multiplexing and Amplification in a Multi Channel Plan," filed by Viet Le on Sep. 4, 1997, assigned to the assignee of the present invention and incorporated by reference herein. Another optical system is described in copending U.S. application Ser. No. 08/672,808 entitled, "System and Method for Photonic Facility and Line Protection Switching," filed by John Fee on Jun. 28, 1996, assigned to the assignee of the present invention, and incorporated by reference herein.
Thus, fiber optic communications links, especially WDM communication links, carry vast amounts of information among distant sites to accomplish data, voice and image connectivity over a large geographical area. Optical transmission lines, transmitters and receivers, however, can fail. The failure of such components can have a substantial economic and practical impact on network users and network service providers. Therefore, in designing communications networks, special measures are practiced to assure utmost reliability of network components and survivability in the event of a failure.
Two types of failures experienced in a telecommunications network are line failures and module failures. A link in a telecommunications network has a transmitter and a receiver, which are also referred to as modules, and a line between the transmitter and receiver. Line failures include damage to the physical fiber and optical component failure, such as the malfunction of amplification equipment situated along the fiber optic cable. Line failures affect the communications line between two network sites. In contrast, a failure of the transmit or receive equipment, such as a laser diode transmitter, housed at either end of an optical communications link is referred to as a module failure. Both line failures and module failures may disable a link between two nodes.
In the event of either a line or module failure, restoration techniques are used to restore the traffic temporarily until the failure is repaired. The restoration approach varies depending on the failure. Traffic may be restored using line protect switching (LPS) or network restorative switching (NRS). If the traffic is restored using LPS, line terminating equipment (LTE) switches the signal from the failed channel to a spare channel within the LTE. If the traffic is restored using NRS, traffic is rerouted by switching the traffic to different routes through the network based on information stored in switch tables or a pre-planned algorithm stored in the switch or a dynamic algorithm which discovers alternate routes at the time of a failure.
LPS is performed strictly within a span. If one traffic-bearing link fails, then the LTE's at each end of the span switch to a protect channel or protect link reserved within the span.
In contrast to LPS, NRS involves rerouting of traffic through a set of nodes in a mesh network and may be used to recover even from failures wherein an entire span is disabled. A technique for accomplishing network restoration is taught by Grover in U.S. Pat. No. 4,956,835. NRS is a means by which spare capacity distributed across the many spans of a network can contribute to the survivability of a span failure. Thus, network survivability is improved while minimizing wasteful redundancy at each span.
LPS ensures resiliency to fiber cuts by employing a spare link, referred to as the protect channel, that normally does not carry traffic but may be used as a back-up should a traffic-bearing link fail. The protect channel is on a different path in order to reduce the likelihood that the protect channel will experience the same failure as the channel in use. Creating and maintaining idle spare capacity is costly. In order to reduce costs, one spare channel is available for restoration of multiple traffic carrying channels. This is called a 1:N or one-to-N protection scheme. When one protect channel is available to restore multiple traffic carrying channels, LPS cannot restore a failure of more than one link. LPS is primarily aimed at restoring single link failures and is implemented within LTE which is the local equipment that terminates the fiber optic cable. Since LPS is localized and simple, it is also very fast requiring only tens of milliseconds for restoring a failed communications link.
Because telecommunications networks include high capacity optical cables such as WDM, the networks are susceptible to failures that disable a very large number of channels and which cannot be restored by LPS alone causing potential high volumes of traffic loss and significant economic impact. Accordingly, NRS is used to restore optical networks. Exchanges have the capability to reroute traffic automatically using switch tables or an algorithm to other transmission paths in the network. Exchanges which have switching capability are connected to LTE at each site and to other transmission paths in the network. When a fiber cut or other major fiber failure occurs disabling a span including a large number of telecommunications links between two switching nodes, NRS is employed by the exchange to reroute the traffic through the restoration network to circumvent the failure until repairs are completed.
Line protect switching (LPS) and network restorative switching (NRS) have separate but complementary roles in a modem network design. The LPS can quickly restore simple localized failures without having to invoke the more complex NRS. In many applications, the LPS can switch traffic without causing any significant interruption to traffic. The NRS can handle network problems of a larger scope.
However, current LPS techniques do not allow for the restoration of failures that involve more than one channel. Although current LPS reroutes the traffic to a spare protect channel, only one protect channel is available to restore multiple channels. Therefore, if the protect channel is in use restoring one link failure, subsequent failures cannot be restored using LPS.
In addition, current NRS does not allow for restoration by components in the optical network via a restoration network.